The present invention relates to planarization of semiconductor wafers using a chemical mechanical planarization technique. More particularly, the present invention relates to a wafer polishing head assembly for use in chemical mechanical polishing/planarization of semiconductor wafers.
Semiconductor wafers are typically fabricated with multiple copies of a desired integrated circuit design that will later be separated and made into individual chips. Wafers are commonly constructed in layers, where a portion of a circuit is created on a layer and conductive vias are created to electrically connect the circuit to other layers. After each layer of the circuit is etched on the wafer, an oxide layer is put down allowing the vias to pass through but covering the rest of the previous circuit level. Each layer of the circuit can create or add unevenness to the wafer that is typically smoothed before generating the next circuit layer.
Chemical mechanical planarization (CMP) techniques are used to planarize the raw wafer and each layer of material added thereafter. Available CMP systems, commonly called wafer polishers, often use a rotating wafer carrier head that brings the wafer into contact with a polishing pad rotating in the plane of the wafer surface to be planarized. A polishing fluid, such as a chemical polishing agent or slurry containing micro abrasives is applied to the polishing pad to polish the wafer. The wafer is pressed against the rotating polishing pad and is rotated to polish and planarize the wafer. Another CMP technique uses a linear polisher. Instead of a rotating pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated to average out the local variations.
The wafer carrier head holds the wafer in place during the polishing operation. In addition, a down force is typically applied to the wafer carrier head to press the wafer into engagement with the polishing pad. The wafer carrier head may also be coupled to a rotating mechanism so that the wafer can rotate while being pressed against a polishing surface. To obtain uniform polishing and planarization of the wafers, the wafer should be maintained generally parallel with the polishing pad.
A known problem can occur when the wafer is not uniformly pressed against the polishing pad or otherwise fails to be maintained generally parallel therewith. The combination of the rotational force and the down force may cause the wafer to tilt downward into the polishing surface. In addition, application of the predetermined force may cause deformation in the wafer carrier head that causes the wafer to be pressed against the polishing surface unevenly. When these conditions occur, nonuniform planarization and/or polishing may occur.
Prior art methods and systems of preventing nonuniform planarization and/or polishing typically involve modifications to the wafer carrier head that are complicated, add considerable weight and require components that involve specialized machining. Accordingly, there is a need for systems and methods of maintaining the wafer carrier head in a plane generally parallel with the polishing pad when the wafer is pressed against the polishing pad that are simple, lightweight and allow relatively simple modification to reflect process conditions.
To alleviate the disadvantages of the prior art, a polishing head assembly is disclosed that includes a head retainer assembly movably coupled to a wafer carrier head. The wafer carrier head is operable to retain a wafer on a bottom surface. The head retainer assembly includes a gimbal post and a load suspension plate that are operable to control the wafer carrier head. Control of the wafer carrier head maintains the wafer carrier head in a plane generally parallel with the polishing pad when a loading force is applied. The loading force is applied to the head retainer assembly to press the wafer into the polishing pad. The head retainer assembly is operable to transfer the loading force to the wafer carrier head using the gimbal post and the load suspension plate.
In addition to transferring the loading force, the head retainer assembly is also operable to optimize the tilt and the deformation of the wafer carrier head. Optimization of the tilt of the wafer carrier head involves using a ball and socket arrangement to allow the wafer carrier head to gimbal with respect to the head retainer assembly. The determination of the optimal location of a gimbal center that effectively cancels a moment force associated with the moving polishing pad optimizes the tilt of the wafer carrier head. When the wafer on the wafer carrier head is brought into contact with the polishing pad, the moment force can cause the wafer carrier head to tilt and unevenly contact the polishing pad. By adjusting the location of the gimbal center based on testing under process conditions, the tilt of the wafer carrier head can be controlled.
The load suspension plate distributes the loading force that is transferred to the wafer carrier head. Control of the distribution of the loading force controls the deformation of the wafer carrier head. Optimization of the flatness of the wafer may be obtained by controlling the deformation of the wafer carrier head. Adjusting the diameter of the load suspension plate controls the deformation of the wafer carrier head and the wafer thereon. The load suspension plate includes a flat circular plate that contacts a region of the wafer carrier head. By adjusting the diameter of the load suspension plate, the region of contact on the wafer carrier head is correspondingly adjusted. Accordingly, the application of the loading force to the wafer carrier head can be controlled to optimize the uniformity of the contact between the wafer and the polishing pad.
Optimization of the tilt and the deformation of the wafer carrier head results in the maintenance of the wafer in a plane that is parallel to the polishing pad when the loading force is applied to the head retainer assembly. Maintenance of the wafer in the parallel plane provides uniform polishing and planarization of the wafer. Accordingly, closer tolerances in the flatness of the wafer can be achieved and consistency of achieving the tolerances can be maintained. The presently preferred wafer polishing assembly is operable to maintain the parallelism of the wafer using the head retainer assembly thereby avoiding complicated modification of the wafer carrier head.